Code refactoring of GC_check_tls_for/GC_check_fl_marks

Code refactoring of GC_check_tls_for/GC_check_fl_marks

* alloc.c (GC_check_fl_marks): Change prototype (pass pointer to
"freelist" element instead of value); do not define the function if
THREAD_LOCAL_ALLOC undefined.
* include/private/gc_priv.h (GC_check_fl_marks): Likewise.
* alloc.c (GC_check_fl_marks): Skip check if the argument points to
a special (non-pointer) value; update comment; rename "q" local
variable to "list".
* thread_local_alloc.c (GC_check_tls_for): Update code according to
GC_check_fl_marks functionality change (remove checks for special
value).